This invention relates to combination intercom/viewing devices for entrances to homes or apartments, and more particularly to devices of this type which are mounted directly to the door to facilitate communication with and visual observation of visitors without opening the door.
By way of background, a typical through-the-door intercom/observation system of the type with which this invention is concerned, includes an elongated viewing tube which is inserted through an aperture in the door as well as through a pair of housings overlying the door aperture at opposite sides thereof. The housings can be either bolted or otherwise secured on opposite sides of the door. For example, threaded elements may be provided at each end of the tube, with the threaded elements being flared such that when they are screwed into the tube the housings are secured in sandwich relation to the door.
Visual observation of visitors by the owner is accomplished without opening the door merely by looking through the tube. To achieve the intercom function and thereby establish audio communication without opening the door, each of the housings is typically provided with a microphone and a speaker, or a combination microphone/speaker element. The electronic circuitry necessary to amplify and receive the audio signals is mounted in one or the other of the housings. Such audio circuitry is generally a simple operational amplifier, and includes a switch which connects a microphone on one side of the door to the input of the amplifier and the output thereof to the speaker on the other side of the door, or vice versa.
A serious problem arises with combined visual/intercom devices of the foregoing type due to the fact that because the housings are bolted together, the speaker in one housing and the microphone in the other housing are spaced apart only a few inches, creating a short acoustic feedback loop. As a consequence, during operation of the intercom, there is a tendency for vibrations to be transmitted directly through the housing and the door from one speaker to the other microphone (and vice versa), thereby creating acoustic feedback. When that occurs, the natural tendency is for the amplifier to "run away" and produce a loud, high-pitched squeal on the speaker. That squeal is irritating and during amplifier runaway, little, if any, audio communication can actually take place.